Sources said Todashev was about to admit his role in a triple homicide that happened in Waltham on Sept. 11, 2011, when he "went crazy." Sources said Todashev pulled a knife and stabbed the FBI agent "several times, ABC reported.

"There was some sort of aggressive movement that led the FBI agent to believe he was under threat and he opened fire," the law enforcement official told ABC News.

In the Waltham incident, three people were found dead in an apartment with marijuana and cash covering their bodies. It was not clear what role Todashev may have played, but he was "about to sign a statement" admitting to a role with Tsarnaev in the unsolved murders.

Taramiv said the men were questioned for hours by the FBI at the Orlando apartment.

"(The FBI) took me and my friend, the suspect that got killed. They were talking to us, both of us, right? And they said they need him for a little more, for a couple more hours, and I left, and they told me they're going to bring him back. They never brought him back," Taramiv said.

He said Todashev spoke with Tsarnaev one month before the April 15 attack, but adamantly denied that Todashev had anything to do with the bombing.

"He felt inside he was going to get shot," Taramiv said about Todashev. "I told him, 'Everything is going to be fine, don't worry about it.' He said, 'I have a really bad feeling.'"

Tsarnaev was shot and killed during a shootout with police in Watertown last month, days after the bombings that claimed three lives and injured dozens of people.

His younger brother, Dzhokhar Tsarnaev, 19, was charged with using and conspiring to use a weapon of mass destruction resulting in death and malicious destruction of property resulting in death in connection with the bombings.